1. Field of the Invention
The invention relates to circuitry for controlling the output illumination level of gas discharge lamps and more particularly to circuitry having load side control and improved lamp current waveforms utilizing a circulating inductor circuit in parallel with a controlled impedance coupled between the ballast and the gas discharge lamps.
Numerous techniques have been proposed for controlling the output illumination level of gas discharge lamps. Present day objectives are directed to efficient energy use, and exemplifying such applications are control circuits for lamp dimming in response to selected illumination levels or varying secondary sources such as natural sunlight. One such system is illustrated in U.S. Pat. No. 4,197,485. Principal deficiencies impeding the development of this technology have been (1) dimming systems have, heretofore, generally reduced the net efficiency (lumen output/wattage input) of the lighting system; (2) the dimming circuitry, when sufficiently sophisticated to provide efficient dimming, becomes costly and burdensome. In contrast, the present invention is directed to a simple, yet efficient, method for illumination control of gas discharge lamps.
An alternative commonly employed to increase overall efficiency in dimming systems is to convert line frequency to higher frequencies. Illustrative of this technique are U.S. Pat. Nos. 4,207,497 and 4,207,498. In contrast, the present invention operates at line frequency. To enhance efficiency, the invention employs a novel configuration of load side control complemented by an inductive circulating current load to achieve circuit simplicity while maintaining an excellent power factor, illumination control of 10 to 1 dimming, excellent current crest factor and reduced lamp current and ballast loss. An attendant advantage of the circuit simplicity is the ready adaptation of the circuit to the physical housing of the conventional gas discharge lamp, an important economic and aesthetic concern.